1. Field of the Invention
This invention relates generally to tape recorders and playback machines and, more particularly, is directed to detecting discontinuities in the time code addresses which are often recorded upon tapes, particularly video tapes, to identify the location of corresponding recorded information signals.
2. Description of the Prior Art
There are many functions performed with tape recorders, particularly video tape recorders (VTRs), in which it is desirable to provide means for accurately addressing a recording tape. Such addressing means makes it possible to easily and accurately determine the position or address of selected signals recorded upon a tape, and subsequently to quickly relocate such selected signals. In the field of broadcast and commercial video tape recording, in which sophisticated machinery has been developed for the control of VTRs, such addresses are used to automatically cause VTRs to cue to, and to initiate and terminate playback at specified tape positions. In video tape editing, such addresses are used to control the relatively complicated task of automatically causing a playback VTR and a record VTR to each locate at a pre-roll position a predetermined number of frames before the respective cut-in points selected separately for the playback and record tapes, to start up, to achieve synchronization with each other, and to simultaneously reach their respective cut-in points at which time the signal being reproduced from the tape in the playback VTR commences to be recorded on the tape in the record VTR. In addition, such addresses are used in video editing to determine when the edit ends, to preview the edit, to initiate slow motion or the operation of special effect generators and to cause various of the signals which can be recorded on a VTR, such as the video signal and one or more audio signals, to be cut in or out at different times.
One method which has been used in the past to address video tapes is the counting of the control pulses or control track signals which are recorded in a track along many video tapes for the purpose of synchronizing the motion of such tapes relative to the moving of rotary transducer heads which are used to record and playback video signals. Since each recording format records a predetermined number of such control pulses in conjunction with each frame, counting of the control pulses can provide an accurate relative address for each frame.
Unfortunately, determining tape addresses by counting control pulses has several major disadvantages. Most important is the fact that the counting of control pulses provides nothing more than a measure of the distance along the tape occupied by the counted control pulses and thus can only provide a tape address that is relative to a point on the tape from which the control pulses have been continuously counted. In order to obtain an absolute tape address, it is necessary to set the counter to zero or another predetermined value at a known location, usually near the start of the tape, and to continuously count control pulses from such known location. If, for any reason, the count value is lost, it is necessary to return to the known location and reset the counter. As the tape is moved over greater distances, and as it is stopped and started and moved alternately in forward and reverse directions, as tapes often are in the editing process, it becomes increasingly likely that an occasional control pulse will be miscounted, owing to drop out, random noise, or the passage of a control pulse past the control pulse reading head at a time when the speed of the tape is too low for the control pulse to be reliably read. Each such miscount adversely affects the accuracy of all subsequently counted tape addresses until the count is again reset.
Because of the foregoing problems associated with determining tape addresses by counting control pulses, professional or broadcast video tape equipment often provides means for recording and reading time code addresses on a video tape which separably identify each frame of the recorded video signal. Such time codes commonly identify each frame of the video signal in terms of an hour, a minute, a second, and a frame number.
At least two types of time codes are used, namely, longitudinal time codes in a track along a longitudinal margin of the tape and vertical interval time codes which are recorded on the same tracks as the video signal, but in the portion of such tracks corresponding to the vertical blanking periods or intervals. It is common for each of these two time codes to designate each frame of a video signal by a corresponding frame number which contains an hour, a minute, a second, and a frame value, each of which values is represented by two decimal digits. For example, the Society of Motion Picture and Television Engineers (SMPTE) time code for NTSC signals varies sequentially from a lowest possible frame number of 00 hours, 00 minutes, 00 seconds, and 00 frames to a maximum possible frame number of 23 hours, 59 minutes, 59 seconds, and 29 frames. In addition, the SMPTE NTSC time code provides room in the so-called "user digits and user bits" for a user to insert data of his choice, such as numbers designating the reel of tape, or the date, on which the recording is made. Such time code frame numbers are provided by electronic equipment known as time code generators, which are either part of, or connected to, a VTR upon which a corresponding video signal is recorded.
Time codes provide an accurate and reliable means for determining tape location. Unlike an address determined by the counting of control pulses, an address determined by the reading of time codes is affected only temporarily by an occasional inability to read a time code, it does not require resetting, and it is not subject to more than a brief loss if a register which is storing the address temporarily malfunctions. For these reasons, it is commmon for professional video tape equipment, particularly video tape editors, to control tape position and tape motion by reference to time code addresses.
Unfortunately, such time code controlled video tape equipment is likely to function erratically if the length of tape on which it operates does not have a continuous sequence of time code addresses, that is, a sequence of time code addresses in which the frame number associated with each successive frame of the recorded video signal is one greater than its predecessor. This is because time code controlled equipment performs those functions which involve moving tape a specified distance by adding or subtracting a number of frames corresponding to such a specified distance to or from a known frame number, so as to derive a calculated frame number, and then by moving the tape until that calculated frame number is read. If there is a discontinuity in the sequence of time code between that identifying the known frame number and the desired point which is the specified distance from it, it is most unlikely that the desired point will have thereon a time code identifying the calculated frame number. In such case, the time code requesting the calculated frame number may be read way before the desired point, way after the desired point, or perhaps not at all, even after searching to the end of the tape.
Similarly, if the desired point occurs on a portion of the tape on which time code addresses have not even been recorded, or from which they cannot be read, for example, because of drop out, the time code requesting the calculated frame number associated with the desired point will probably never be read, thereby causing the equipment to continue searching for the calculated time code all the way to the end of the tape.
Time code discontinuities of the type which cause the above-mentioned problems often occur in video recording. If, as is often done, a plurality of independent or separate video sequences or segments are recorded on a tape at different times, the time code addresses on the tape will be discontinnuous between successive video segments, that is, the time code addresses at the end of one segment and at the beginning of the next segment, respectively, are likely to vary by an undefined amount and/or to have gaps between them in which no time codes are recorded. Similarly, time code discontinuities result when video sequences from different tapes are recorded one after the other on one tape in the process of assemble editing. Discontinuities also result when a video sequence having no accompanying time codes is recorded on a tape by assemble editing next to a video sequence having time codes.
In order to prevent the above difficulties caused by time code discontinuities, it is common to assemble recorded video tape signals onto a new tape using a so-called jam sync time code generator which provides a new sequence of time codes for the assembled tape, with the time code at the start of each successive video sequence edited onto the tape having a frame number one higher than that at the end of the previous sequence. Unfortunately, such jam sync generators are complicated and expensive, and, since they record the respective time code in place of any time codes which may have been recorded on the original tapes from which assembled video sequences come, they cause the time code information recorded on such original tapes to be lost. This is undesirable because the time code originally recorded with a video sequence, in addition to indicating the time of day that such a video sequence was recorded, can contain, in the user digits and bits, information which makes it possible to determine the tape reel, or the date, on which such a video signal was recorded.
A Japanese published unexamined patent application No. 16538/1978, filed July 29, 1976, and which was laid open to the public on Feb. 15, 1978, and has a common assignee with this application, discloses a time code address reader for use in video tape equipment and containing a dropout compensation circuit for time codes. In such time code address reader, current time codes are read from a tape into a time code register, means are provided for detecting when time codes can and cannot be read, and the value in the time code register is incremented or decremented in response to control pulses which are read during periods when time codes cannot be read as a result of drop out. Although such disclosure does provide means for maintaining an accurate time code address during brief periods in which the time code cannot be read, for example, as a result of drop out, it fails to provide means for detecting or providing information about a discontinuity in which two adjacent time code addresses have non sequential frame numbers, as results from the assembling, without a jam sync time code generator, of video sequences originally recorded at substantially different times.